Effects of coordinating heteroatoms on molecular structure, thermodynamic stability and redox behavior of uranyl(vi) complexes with pentadentate Schiff-base ligands.

Uranyl(vi) complexes with pentadentate N 3 O 2 -, N 2 O 3 - and N 2 O 2 S 1 -donating Schiff base ligands, t Bu,MeO-saldien-X 2- (X = NH, O and S), were synthesized and thoroughly characterized by 1 H NMR, IR, elemental analysis, and single crystal X-ray diffraction. The crystal structures of UO 2 ( t Bu,MeO-saldien-X) showed that the U-X bond strength follows U-O ≈ U-NH > U-S. Conditional stability constants ( β X ) of UO 2 ( t Bu,MeO-saldien-X) in ethanol were investigated to understand the effect of X on thermodynamic stability. The log  β X decrease in the order of UO 2 ( t Bu,MeO-saldien-NH) (log  β NH = 10) > UO 2 ( t Bu,MeO-saldien-O) (log  β O = 7.24) > UO 2 ( t Bu,MeO-saldien-S) (log  β S = 5.2). This trend cannot be explained only by Pearson's Hard and Soft Acids and Bases (HSAB) principle, but rather follows the order of basicity of X. Theoretical calculations of UO 2 ( t Bu,MeO-saldien-X) suggested that the ionic character of U-X bonds decreases in the order of U-NH > U-O > U-S, while the covalency increases in the order U-O < U-NH < U-S. Redox potentials of all UO 2 ( t Bu,MeO-saldien-X) in DMSO were similar to each other regardless of the difference in X. Spectroelectrochemical measurements and DFT calculations revealed that the center U 6+ of each UO 2 ( t Bu,MeO-saldien-X) undergoes one-electron reduction to afford the corresponding uranyl(v) complex. Consequently, the difference in X of UO 2 ( t Bu,MeO-saldien-X) affects the coordination of t Bu,MeO-saldien-X 2- with UO 2 2+ . However, the HSAB principle is not always prominent, but the Lewis basicity and balance between ionic and covalent characters of the U-X interactions are more relevant to determine the bond strengths.